Shorted dual element magnetoresistive reproduce head exhibiting high density signal amplification

ABSTRACT

An MR head includes two substantially identical MR elements, separated by an insulating layer which has shorting stubs at its ends for electrically shorting the MR elements. A current applied to the MR elements splits into two currents that flow in the same direction through the substantially identical MR elements, to provide mutual bias and to serve as sense currents for detecting change in element resistance. The MR elements are biased to operate in a magnetically unsaturated mode. This results in a &#34;bootstrapping&#34; of short wavelength signals that effectively amplifies the reproduced signal over a broad region of the signal spectrum when the linear spacing between the MR elements is in the range of from one half to one times the half-wavelength of signals recorded on a magnetic recording medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a magnetoresistive reproduce head, and inparticular to a dual element magnetoresistive head.

2. Description Relative to the Prior Art

The magnetoresistive (MR) reproduce head has gained wide acceptance inthe magnetic recording field since it was disclosed in U.S. Pat. No.3,493,694, issued to Hunt in 1970. The MR head is characterized by highoutput and low noise, making it particularly attractive for reproducingshort wavelength signals. It may be fabricated by thin film depositiontechniques allowing the relatively inexpensive manufacture of multitrackheads with narrow track widths for high density recording applications.A variety of shielded and unshielded configurations using single anddual MR elements and incorporating a number of biasing techniques areknown in the art.

Dual element MR heads are disclosed in U.S. Pat. No. 3,860,965 entitled"Magnetoresistive Read Head Assembly Having Matched Elements for CommonMode Rejection", issued in the name of Voegli and U.S. Pat. No.4,878,140 entitled, "Magnetoresistive Sensor with Opposing Currents forReading Perpendicularly Recorded Media", issued in the names of Gill etal. The heads disclosed in these patents comprise parallel MR elementsseparated by thin electrically insulating layers. It has long been knownin the art, that MR structures such as the above, whose elements areseparated by thin electrically insulating spacers are subject toshorting problems. Such shorting may be due to pin holes in theinsulating spacer, or may occur in head lapping, or during headoperation when the abrasive magnetic tape being reproduced can smear thesoft MR element across the spacer, shorting it to adjacent conductivematerial. This has occurred, for example, in heads utilizing softadjacent layer biasing where an MR element is separated by an thinelectrically insulating spacer from a conductive magnetic material whosemagnetic field induces the bias in the MR element. Bajorek et al,recognizing the problem in their U.S. Pat. No. 4,024,489 entitled"Magnetoresistive Sandwich including Sensor Electrically Parallel withElectrical Shunt and Magnetic Biasing Layers", teach overcoming theproblem by intentionally shorting the MR sensing layer and the magneticbiasing layer by use of a very thin (220 angstrom), contiguousconductive separation layer. However, the structure disclosed by Bajoreket al, is stated to result in a 30% loss of signal from the single MRsensor for a given energy dissipation in the head because the currentflowing through the conductive shunting layer provides no contributionto signal output.

SUMMARY OF THE INVENTION

In a preferred embodiment, the MR dual element head of the presentinvention solves the shorting problem between two MR elements,(whichserve both as sensing elements and mutual biasing elements), withoutsuffering the penalty of signal reduction due to shunting of the sensecurrent. The two identical MR elements are neither totally insulatedfrom each other, nor shorted along their entire lengths by a contiguousconductive spacer, but are separated by an insulating layer which hasshorting stubs at its ends for electrically shorting the MR elements. Acurrent applied to the shorted MR elements splits into two equalcurrents that flow in the same direction through the substantiallyidentical MR elements to provide the bias, and to serve as sensecurrents for detecting element resistance change. No sense current isshunted through the spacer. If a short does occur in the insulation ofthe spacer, no current flows through the short between the matched MRelements because the current flows in the same direction in eachelement, and each element is matched to have the same resistance. Thereis no voltage difference across the short, and therefore, shorts betweenthe MR elements do not interfere with detection of recorded signals.

Additionally in the practice of the invention, the MR elements arebiased to operate in a magnetically unsaturated mode. This results in a"bootstrapping" of short wavelength signals that effectively amplifiesthe reproduced signal over a broad region of the signal spectrum. Thedesign criterion for determining the amplified portion of the spectrumcalls for a linear spacing between the MR elements in the range of fromone half to one times the linear distance between the flux changesrecorded on the signal medium. Over this range of spacer thickness, theamplified response is relatively insensitive to the separation. For ashort wavelength flux density of 80 kiloflux changes per inch, thespacer is typically selected to have a thickness of about 1500angstroms.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to the figures, of which:

FIG. 1 is a drawing of a dual MR head according to the invention, incontact with a recorded magnetic tape,

FIG. 2 is a schematic drawing of the resistances of the MR elements ofthe head of FIG. 1,

FIG. 3a illustrates the mutual biasing of the MR elements of the head ofthe invention,

FIG. 3b is a plot of the fractional change in resistance of theoppositely biased MR elements as a function of applied magnetic field,

FIG. 4 is a plot of the fractional change in resistance of the MRelements of the head of the invention for applied signal,

FIGS. 5a, 5b, 5c illustrate the signal amplification effect exhibited bythe head of the invention,

FIG. 6 is a plot of the output signal level as a function of recordedflux density for the head of the invention compared to that of anunshielded MR head, and

FIG. 7 is a drawing of a second embodiment of the head of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of the invention, the MR elements areelectrically shorted to each other at their longitudinal ends, but areseparated by an electrically insulating spacer for the rest of theirlengths. Referring to FIG. 1, a dual MR head 70 consists of twomagnetically, electrically and geometrically matched MR elements 72,74.The MR elements 72,74 are separated over substantially their entirelengths by a non-conductive spacer 76. At the longitudinal ends of thespacer 76 are two electrically shorting stubs 78,80, of the same widthas the spacer 76, which electrically short the two MR elements to eachother. A sense and biasing current 82 flowing to the head 70 via theleads 84, 86 divides between the two MR elements 72,74 into the equalcurrents 88,90, because the MR elements 72,74 are identical, and becauseof the electrically shorting stubs 78,80. Referring to FIG. 2, theequivalent electrical circuit of the structure of the MR head 70 is seenwith the current 82 flowing into the parallel resistances R₇₂, which isthe resistance of MR element 72, and R₇₄ which is that of the MR element74. Assuming an inadvertent electrical short 92 occurs between the MRelements 72, 74, due, say, to a pin hole in the insulating spacer 76.Because of the substantially identical characteristics of the MRelements 72, 74 and the equality of the currents 88, 90, the voltagese₁, e₂ along the length of each of the MR elements 72,74 are equal. Thatis, no current flows in the electrical short 92, and the distribution ofcurrents in the MR elements is unchanged by the electrical short 92.Therefore, the magnetic biasing and the signal performance, which arefunctions of the sense currents 88, 90, are operationally immune to thepresence of shorts.

The thin film MR elements 72,74 are rectangular in shape. Thisconfiguration results in the shape anisotropy of the MR film being alongthe longitudinal axis of the film, which is also the direction of theunbiased magnetization of the film. As will be explained below, the biasrotates the magnetization from this axial direction, and the signalsfrom the medium further modulate the position of the magnetization,changing the film resistance. It will be noted that the signal fieldsfrom the tape 30 are directed along the short dimensions of the MRelements 72,74. The longitudinal width of the sandwich is generallyequal to or slightly less than a track width of the data recorded on thetape 30. The values of the head parameters are determined by theapplication. For example, in a head having a 50 micron trackwidth andoperating at 80 kiloflux changes per inch, suitable parameters are: thewidths of the MR elements 72, 74 and the spacer 76 equal to 50 microns,the MR element 72,74 thicknesses equal to approximately 250 angstroms,the thickness of the spacer 76 equal to 1500 angstroms, and the heightsof the MR elements 72,74, and of the spacer 76 equal to 5 microns.

The MR head 70 is seen in contact with a magnetic tape 30 havingalternately magnetized portions 32, 34, 36, 38, 40, throughout thelength of the tape 30, which comprise the information recorded on thetape 30. A wavelength of the recorded signal on the tape 30 encompassestwo contiguous, oppositely directed magnetized regions, for example, theregions covered by the arrows 32 and 34. The number of alternatingmagnetized portions 32, 34, 36, 38, 40 per inch is the number of fluxchanges per inch recorded on the tape 30.

Referring to FIG. 3a, the currents 88, 90, flowing in the same directioninto the MR elements 72,74 generate the magnetic fields that result inthe mutual biasing of the elements 72,74, because each of the MRelements, as well as being field detection elements, acts as a softadjacent biasing layer for the other. As the elements 72,74 aremagnetically and geometrically the same, and because the amplitudes ofthe currents 88,90 are the same, the bias field H_(B) at element 72 dueto the soft adjacent biasing layer action of the MR element 74, is equalin magnitude and opposite in sign to the bias field -H_(B) at element 74due to the soft adjacent biasing action of the MR element 72. As isknown in the art, in biasing the MR elements 72,74, the magnetic fieldH_(B) rotates the magnetization of the MR element 72 in one direction,and the field -H_(B) rotates the magnetization of the MR element 74 anequal amount in the opposite direction.

Referring to FIG. 3b, the curves 42,42' are the symmetrical, change ofresistance vs. magnetic field curves for the biased magnetoresistiveelements 72,74. As described above, the bias fields in the elements72,74 have the same amplitudes but opposite signs, and the elements72,74 themselves are magnetically matched. Therefore, the curves 42,42',(which are arbitrarily assigned to the MR elements 72 and 74respectively,) are substantially identical, and are symmetricallyshifted with respect to the origin by the applied bias fields. Thehorizontal axis of FIG. 3b being the applied signal field, H_(s), itwill be seen that with no applied signal field, the quiescent bias point44 is symmetrically positioned on the oppositely sloping sides of thecurves 42,42'.

U.S. Pat. No. 4,833,560, "Self-Biased Magnetoresistive Reproduce Head"issued in the name of Doyle, and assigned to the same Assignee as theinstant application, teaches orienting the induced anisotropy fields ofan MR element and a biasing adjacent layer so that their inducedanisotropy fields lie in the same directions along the short dimensionof the rectangular magnetoresistive elements. It will be recalled in thepresent invention that each of the MR elements 72,74 acts as a softadjacent layer for the biasing the other element in addition to its roleas a signal detector. The induced anisotropy fields of the MR elementsof the present invention can be made to lie in the direction of the biasfields at the MR elements, i.e., along the short dimensions of the MRelements 72,74 as taught by U.S. Pat. No. 4,833,560, which is herebyincorporated by reference.

Referring again to FIG. 1, it will be seen that the depicted wavelengthrecorded on the tape 30 is such that a half wavelength is equal to theseparation between the MR elements 72,74. Under this condition themagnetic fields from the tape 30 at the elements 72,74 are 180 degreesout of phase. In FIG. 4, the change in resistance vs. magnetic fieldcurves 42,42' is again shown, along with a signal field 48 applied tothe MR element 72 (curve 42 applies), while a signal field 50 is appliedto the MR element 74 (curve 42' applies). The wavelength of the signals48,50 is equal to twice the separation of the MR elements 72,74 andtherefore the signals 48,50 are 180 degrees out of phase. The signals48,50 swing the resistance of the MR elements 72, 74 about the biaspoints 44 and the change of resistance for the MR element 72 is depictedby waveform 52, while that for the MR element 74 is depicted by waveform54. It will be appreciated that the output signals derived from theabove changes in resistance are in phase in the two electricallyparalleled MR elements, and therefore the resultant output signalvoltages due to the sense currents 88,90 are also in phase.

The operation of the invention in effecting amplification of a shortwavelength reproduced signal may be understood by referring to FIG. 5a,FIG. 5b, and FIG. 5c, wherein the parts played by the magnetic field ofthe recorded medium, the induced magnetization in the MR elements, andthe induced fields in the MR elements are shown. (The events portrayedin these figures actually occur simultaneously, and all fields arepresent at the same time. In the figures they are shown occurring insequence for clarity.) In FIG. 5a, a section of the magnetized medium 30is illustrated passing under the MR element 72 and MR element 74.Positive magnetization 36 (arbitrarily defined as pointing to the leftin FIG. 5a) and negative magnetization (in the opposite direction) 34recorded in the medium 30 give rise to a signal field H_(S). FIG. 5aillustrates the condition where the distance between the transitionsfrom positive magnetization to negative magnetization in the medium 30approximately equals the separation distance between the MR element 72and the MR element 74. This is the condition for signal amplification;however, as previously noted, the response is relatively insensitive tospacer thickness when it is in the range of from one half to one timesthe distance between transitions. As shown in FIG. 5a, part of thesignal field H_(S) threads the low magnetic reluctance path through theMR element 72 and MR element 74. Not shown in FIG. 5a, but still presentand essential to the operation of the device, are the static fieldsrelated to the bias as previously discussed. The field H_(S) shown inFIG. 5a is a dynamically incremental field due to the magnetization inthe medium. The H_(S) field in traversing the magnetically softmaterials comprising the MR elements induces magnetization M₇₂ in the MRelement 72, and M₇₄ in the MR element 74. The induced magnetizations,M₇₂ and M₇₄ are also dynamically incremental since they arise from thesignal field H_(S). Because both the MR's 72 and 74 are operating on thelinear portions of their magnetization curves, it will be appreciatedthe magnitude of the induced magnetizations M₇₂ and M.sub. 74 aredirectly proportional to the strength of the field H_(S).

Referring now to FIG. 5b, the magnetization M₇₄ in the MR element 74 bythe signal field H_(S) of FIG. 5a is shown, but the field lines of thegenerating field H_(S) are omitted for clarity. The inducedmagnetization M₇₄ of the MR element 74 in FIG. 5b gives rise to a fieldH₇₄. The flux lines from the field H₇₄ extend to, and are interceptedby, the MR element 72. The intercepted flux from H₇₄ induces additionalmagnetization M_(72') in the MR element 72. It will be appreciated thatthe direction of the field H₇₄ is downward at the MR element 72, andagain referring to FIG. 5a it is seen that H₇₄ is attendantly in thedirection to reinforce the field H_(s) which originally gives rise tothe field H₇₄. Thus the field H₇₄ further modulates the angular positionof the magnetization vector of the MR element 72 and further changes theMR element's 72 magnetoresistance. Referring to FIG. 5c, the inducedmagnetization M_(72') of the MR element 72 also results in a field,H_(72'), flux lines of which are, in turn, intercepted by the MR element74. The field, H_(72'), is upward at the MR element 74, and againreferring to FIG. 5b it will be noted that the field H_(72') reinforcesthe magnetization M₇₄ further increasing the field H₇₄. This"bootstrapping" action between the two MR elements, and the signal fromthe medium provides increased output signal from the MR elements for agiven intensity of magnetization of the medium.

When the medium moves 1/2 the signal wavelength, i.e. by the distance ofone signal flux change relative to the head, the magnetization in themedium below the MR element 72 and the MR element 74 are of oppositesigns to those described above. It will be appreciated that resultantlythe directions of all the induced fields and magnetizations also changesigns, and the overall effect is the continued reinforcement of thesignal field as described above. The "bootstrapping" again augments theeffect of the signal field Hs at the MR element 72, and amplificationthus takes place for both signs of the alternating signal magnetic fieldof the medium.

Referring to FIG. 6, curve 100 is a plot of the response of an dualelectrically shorted MR head in accordance with the invention and, forcomparison, curve 102 is the corresponding response of an unshieldedsingle MR element head. The head of curve 102 is known in the art asconsisting of an MR element which is biased from an external fixed biassource, such as a permanent magnet. A comparison of the curves 100,102shows the improvement obtained at short wavelengths with a dual MRelectrically shorted head.

As previously described, the amplification at shorter wavelengths ariseswhen the separation of the flux changes on the medium is of the sameorder of magnitude as the distance between the MR elements. As thespacing between the flux transitions increase in length, the response ofthe head of the invention slowly decreases, with a drop in amplitudewhen the flux length becomes so long that both of the MR elementssimultaneously "see" a signal of the same polarity from the medium.

A second embodiment is illustrated in FIG. 7, wherein a dual elementmagnetoresistive reproduce head 10 comprises sensing and mutuallybiasing magnetoresistive elements 12, 14, matched for magnetoresistivecharacteristics, electrical resistivity, and geometrical shape anddimensions. The elements 12, 14 are mated with an electricallyconductive, non-magnetic spacer 16 between the element 12, 14 in asandwich configuration. A current 22, which is the sense current andalso the excitation current for biasing the elements 12, 14, flows inthe two leads 18, 20 connected to the sandwich.

The components of the sandwich are in electrical contact for theirentire lengths, and will therefore share any current flowing in thesandwich depending upon their relative resistances. Because themagnetoresistive elements 12,14 are matched for electricalcharacteristics, (as well as magnetic characteristics,) and because ofthe symmetry of the sandwich, the current 22 will divide into componentcurrents 24,26,28 where the currents 24,26 flowing in the same directionthrough the MR elements 12,14 are equal in magnitude, and the remainderof the current 22, i.e. the current 28, flows in the spacer 16.

In this embodiment, the presence of the conductive spacer 28 obviatesthe shorting problem. However, in comparison to the head 70 of FIG. 1,the current 28 shunted through the spacer 16 does not contribute tosignal detection, and for equal power dissipation the head 10 is not asefficient as the head 70. The head 10 exhibits amplificationcharacteristics similar to those shown in FIG. 6 for the head 70. Asseen in FIG. 7, the signal from the tape is also applied in thedirection of the short dimension of the MR elements 12,14, and it istherefore advantageous to orient the induced easy axis along thisdimension, as previously described for the head 70.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A magnetic head assembly for detectingmagnetically recorded signals comprising:a) a first rectangular thinfilm magnetoresistive element having a first long dimension and a firstshort dimension, b) a second rectangular thin film magnetoresistiveelement having a second long dimension and a second short dimension,wherein said first long dimension equals said second long dimension andsaid first short dimension equals said first short dimension, wherebysaid first and said second rectangular thin film magnetoresistiveelements are geometrically congruent, c) a planar non-magnetic thin filmspacer having a third long dimension less than said first and secondlong dimensions, said third long dimension terminating in first andsecond longitudinal ends, and said spacer being in contact with saidmagnetoresistive elements for separation of said first and said secondmagnetoresistive elements, said spacer further comprising anelectrically insulating planar mid-portion, and first and secondelectrically conductive stubs at said first and second longitudinal endsrespectively of said third long dimension, whereby said first and secondelectrically conductive stubs at said longitudinal ends of said spacerprovide electrical shorting between said first and said secondmagnetoresistive elements, d) means for longitudinal concurrent currentflow in said first and second elements, whereby said elements mutuallymagnetically bias each other, e) means for coupling magnetic signalfields from said magnetically recorded signals to said first and saidsecond rectangular magnetoresistive elements along said short dimensionsof said first and second magnetoresistive elements, whereby said firstand said second magnetoresistive elements are concurrently responsive tosaid signal fields, and f) means for detecting the resistance change insaid magnetoresistive elements in response to said signal fields.
 2. Themagnetic head assembly of claim 1 wherein said first and said secondmagnetoresistive elements are unsaturatedly biased.
 3. The magnetic headassembly of claim 2 wherein said first and said second magnetoresistiveelements have substantially matched magnetic and electricalcharacteristics.
 4. The magnetic head assembly of claim 3 wherein saidfirst and second magnetoresistive elements have first and second inducedanisotropy axes along said short dimensions of said rectangularmagnetoresistive elements respectively,
 5. The magnetic head assembly ofclaim 3 wherein said current means is means for providing equalamplitude currents flowing in said first and said secondmagnetoresistive elements.
 6. The magnetic head assembly of claim 5wherein said means for coupling signal fields is means for concurrentlycoupling alternating direction signal fields to said first and saidsecond magnetoresistive elements respectively, whereby said alternatingfields cooperate with said first and said second magnetoresistiveelements to provide means for amplification of said magneticallyrecorded signals being reproduced by said magnetoresistive headassembly.
 7. The magnetic head assembly of claim 6 wherein said meansfor amplification is means for spatially separating said first and saidsecond magnetoresistive elements by a distance in the range of from onehalf to one times the spatial distance between said alternating signalfields.